Compact 2×4 Filtering Butler Matrix Based on Balun Filter

A novel 2×4 filtering Butler matrix (FBM) is designed based on two balun filters and one 3 dB directional coupler. Compared with the traditional Butler matrix, the topology of the FBM avoids the using of the phase shift and cross-over, which efficiently reduces the circuit size. Furthermore, the balun filters introduce the inherent frequency selectivity. The balun filters and the coupler are discussed separately. The distribution of the two input ports as well as the four output ports makes it easy to connect with the external circuits. A prototype of the FBM is given with a circuit size of 0.62λ
g
×0.40λ
g
. The feasibility of this the method is confirmed by the simulated results.